1. Field of the Invention
The present invention relates to an active matrix display device and a driving method thereof. In particular, the invention relates to a display device having a switching element such as a thin film transistor (hereinafter referred to as TFT) and a light-emitting element in each pixel and the driving method thereof. Further, the invention relates to an electronic appliance using the display device and the driving method thereof.
2. Description of the Related Art
In recent years, the technique for forming TFTs has largely progressed, and is applied and developed to active matrix display devices. Particularly, since the field effect mobility (also referred to as mobility) of a TFT using a polysilicon film as an active layer is higher than that of a conventional TFT using an amorphous silicon film, high-speed operations are enabled. Therefore, a pixel can be controlled by a driver circuit which is formed using a TFT over the same substrate as the pixel. A display device in which various circuits are formed using a TFT which is formed over the same substrate as the pixel has various advantages such as a reduction in manufacturing cost, downsizing, higher increase in yield, and higher throughput.
An active matrix EL display device having an electroluminescence element (hereinafter referred to as EL element) that is a light-emitting element as a display element included in each pixel of a display device has been studied actively. An EL display device is also referred to as an organic EL display (OLED) or an organic light-emitting diode (OLED).
In general, since luminance of an EL element is proportional to a current value of the EL element, an EL display device using an EL element as a display element controls the luminance by a current value. Further, there is also a driving method of controlling the luminance by using the time in which a current flows into an EL element under the condition that a current value of the EL element is constant.
Since the luminance of an EL element is proportional to a current value of the EL element, an EL display device using an EL element as a display element has a problem of off current of a switching element in the case where a power source line is connected to the EL element through the switching element. In order to solve this problem, Patent Document 1 is disclosed.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-323873
If a certain pixel of a display device continuously emits light regardless of a video signal, that is, even if one luminescent spot is generated, the display device is often determined to be defective. On the other hand, if a certain pixel always emits no light regardless of a video signal, that is, if a few dark spots are generated, a display device is not determined to be defective.
Further, a luminescent spot is generated because of such a factor that a short circuit is caused between a source and a drain of a TFT connected to an EL element in series, in which case, potentials of a source electrode or a drain electrode of the TFT has the same level, and an EL element continuously emits light regardless of a potential of a gate electrode of the TFT.